This invention relates to data processing technique and more particularly to technique which is effective when applied to microcomputers. The present invention relates to a technique which can be utilized effectively, for example, for an output system of an external sync signal in a single-chip microcomputer having a sync signal.
In conventional single-chip microcomputers such as Model "HD6301V", a product of Hitachi, Ltd., a pin is provided for exclusively outputting an external sync signal called an "E clock" to peripheral devices. This single-chip microcomputer HD6301V includes therein a program storage ROM (read-only memory) and a working RAM (random access memory) and operates by itself, or in other words, a microcomputer system is constituted and operates inside the chip. Besides such a single-chip mode, the microcomputer has an external expansion mode in which the chip constitutes integrally a microcomputer system in cooperation with peripheral devices such as ROMs and RAMs that are connected thereto in order to expand the address space.
In this external expansion mode, the E clock described above is outputted from the single-chip microcomputer to the outside as a sync signal for the exchange of signals between the single-chip microcomputer and the peripheral devices. In the single-chip mode where the peripheral devices are not connected, therefore, the external sync signal (E clock) is not necessary.
However, in the conventional single-chip microcomputers of the type in which the external sync signal is generated inside them, the external sync signal is outputted to the output pin not only in the external expansion mode, but also in the single-chip mode.
In this case, there occurs the problem that noise develops in the signal supplied to a pin, which is disposed next to the E clock output pin, because adjacent external terminals (pins) have an undesirable coupling capacity. In the single-chip microcomputers, such as HD6301V in particular, the pin adjacent to the E clock output pin receives an input strobe signal SC as an input signal in the single-chip mode. Therefore, noise develops in this input strobe signal SC through the coupling capacity, so that erroneous data is likely to be latched.
Furthermore, an output buffer circuit which outputs the E clock as the external sync signal is constituted by circuit elements having greater sizes than other circuit elements because it must provide a great driving force, and the current consumption becomes great.
The single-chip microcomputer HD6301V having the E clock as the external sync signal is described on pages 279 to 318 of "HITACHI MICROCOMPUTER DATA BOOK, 8-BlT SINGLE-CHIP" published by Hitachi, Ltd., February, 1985.